28(Demon Slayer)

An Au where the Tomioka and Kanroji families have noble roots and since many nobles families have died(due to Muzan and his demons, both being killed by them or turned), the current emperor ordered those distante descendants to marry and start a new generation of the almost lost noble families.

Tsukato had already been married(she didn't die but a demon did attack them, they were just lucky a demon slayer had been visiting a friend/family and killed the demon...Giyuu due to what happened, closed himself a bit as it was a traumatic event but nothing like canon) by the time this was ordered so Giyuu was the one who had to fulfil this as the only alive and single member of the Tomioka family despite being only 14, he had been taken to meet different girls from different families to choose a future wife.

But Giyuu just couldn't connect with any of them and he almost believed he would have to just randomly choose one until they visited the final family, the Kanroji family, where he met their currently only daughter(Mitsuri will be just a few months younger then Giyuu for this) who was a ray of sunshine but did not try to force him to talk or anything like the other girls did so he made the decision that if he had to marry someone then it would be Mitsuri(and it helped that her bright hair seemed to fit her personality and her strength added protection that he wasn't sure he could give her since he was never that strong due to having been a premature baby and a rather sick as a baby).

So the two of them marry, and move in with Mitsuri's family until they are 18 when the emperor has them move to the abandoned house of old noble family which both are far descendants from.

They first started as friends(since they were children and while the emperor wanted to further the noble families, he wouldn't have children procreate so soon) but they slowly fell in love.

When Giyuu told Mitsuri about why he was more closed off and was scared of the dark, he had thought Mitsuri would believe him to be crazy but she believed them and decided that she would become a demon slayer to protect him of demons so he would never have to suffer again.

Mitsuri somehow meet Kyojuro, becomes his student and all that and at 20 she becomes the Love Hashira(Sabito would be alive and be the Water Hashira) being welcomed by the otehr hashiras especially as she reminded them of Kyojuro.

Despite sometimes hanging out with her fellow hashiras and being pretty close to the Rengoku family, none of them really knew that she was married until they wanted to surprise her and went to visit the Love Estate only to be told that while Mitsuri stayed there after long missions, she lived in capital most of the time.

That made them all(especially Obanai who still has a crush on her) curious to why Mitsuri would live on the capital as even those with alive family, separated themselves from them(unless they were a family of demon slayers or their living family were demon slayers) so they would be safe.

Uzui and Kyojuro threw around many theories but none of them really had the courage to ask Mitsuri as it was still her private life and she wasn't obligated to tell them about it.

But one day, during a hashira meeting they were interrupted by shouts and a pretty young man entering the garden who apologized to Kagaya about the interruption but before he could say what he was doing there, Mitsuri ran to the man and started asking a million questions about his health, if something had happened to their families, etc.

The hashiras and Ubuyashiki family could only watch as the man answered all the questions as if used to them until Mitsuri calmed down and finally let him talk to wish the man said he had found two hurt kids in the forest near their home(so Tanjiro and Nezuko will both be mostly human, Nezuko still ended up drinking a bit of Muzan's blood but it affected her in the way it should have affected Muzan when the doctor gave him the antidote...but she kept her super strengths, a few demon traits like the teeth and can use a dba) who he was able to get to tell him that their family was attacked by a demon that by what Nezuko could remember, looked human except for his unnatural cat red eyes.

This gathered Kagaya's attention as there was only demon who fit that description, so he asked Giyuu to tell them the story from the beginning and later requested to meet the children once they were healed and feeling better.

At the end of this reunion, despite the news of Muzan appearing, the hashiras couldn't stop themselves from asking Mitsuri for her relationship with Giyuu and were shocked(and broken hearted when it comes to certain serpent hashira) when informed that Giyuu was her lovely husband.

How anything else would go from here, including if the Kamados will become demon slayers, how Giyuu not being and hashira/demon slayer(and Sabito being alive) will affect events, if Kyojuro(and Mitsuri) will die, if MitsuGiyuu have children, etc is up to the writer.